1. Field of the Invention
The present invention relates to a braking fluid pressure control apparatus and more particularly to the braking fluid pressure control apparatus in which a part of wheel brake is operated by pressure generated in a hydraulic booster utilizing the pressure of an auxiliary power unit and other wheel brakes are operated by the hydraulic pressure of a master cylinder. The apparatus secures a favorable braking efficiency when a line for introducing the pressure of the auxiliary power unit to a boost chamber or a line for introducing the boost pressure into a pressurizing unit not operating properly and accomplishes a favorable relationship between the stroke of a brake pedal and braking efficiency when the apparatus is normally operating.
2. Description of the Related Arts
A conventional braking system of an automobile comprises an auxiliary power unit for storing hydraulic fluid sucked from a reservoir by a pump and pressurized; and a hydraulic booster utilizing the pressure of the auxiliary unit. The hydraulic booster is used in place of the conventional vacuum type booster. The hydraulic booster facilitates miniaturization of the braking system, i.e., mounted in the automobile by holding the master cylinder and an unlocking control device together as a unit and facilitates the improvement of braking efficiency.
In addition to the above type braking system, the following system is known: That is, the boost pressure of the hydraulic booster increases the thrust of the master cylinder generated by force applied to the brake pedal and instead of static fluid generated by the master cylinder, dynamic fluid having boost pressure generated in a booster chamber is used to pressurize hydraulic fluid communicating with a part of wheel brakes.
According to a mechanism for controlling the pressure of a brake operating cylinder disclosed in Japanese Patent Laid-Open Publication No. 57-104449, hydraulic fluid communicating with a front wheel brake is pressurized by the master cylinder and hydraulic fluid communicating with a rear wheel brake is pressurized by boost pressure. The mechanism comprises a fluid introducing valve for opening and closing a path communicating a boost chamber and the rear wheel brake with each other and a discharge valve for discharging dynamic fluid from the rear wheel brake to a reservoir. Thus, an unlocking device utilizing dynamic fluid for the rear wheel is provided. In this construction, the unlocking device for the rear wheel can be constructed quite simply because the master cylinder is not operated unefficiently.
However, in the above-described pressure control mechanism, hydraulic fluid communicating with the rear wheel brake does not function and the function of the booster is not effectively used when the auxiliary power unit or the rear wheel brake circuit fails. If the boost function cannot be used, the force for operating the brake pedal necessary for obtaining a required braking efficiency increases, that is, even though the brake pedal is operated at a certain force, the braking efficiency is reduced.
In order to reduce the pedalling force necessary for obtaining a required braking efficiency in this state, the master cylinder diameter needs to be reduced. As a result, the pedal stroke becomes large in order to obtain the required braking efficiency.